The present invention relates to an automotive steering control system, including an assistant steering mechanism for providing assisting force for operating a main steering mechanism, such as a rack-and-pinion steering mechanism. Specifically, it relates to a steering control system capable of providing a stable assisting force for the main steering mechanism when an anti-skid brake control is active. More specifically, the invention relates to a steering angle control system for a four wheel steerable vehicle, for which an anti-skid brake control system for controlling vehicular braking behavior is facilitated.
1. Field of the Invention
Recently, there has been proposed and developed an automotive steering control system employing an assistant steering mechanism which is comprised of a power assist cylinder. As is well known, such a power assist cylinder is traditionally employed in four wheel steering systems.
One example of four wheel steering system has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 59-143770. Such prior proposed steering system controls toe angle of rear wheels as subsidiary steerable wheels on the basis of a vehicle speed and a steering angle of front wheels as primary steerable wheels. Namely, toe angle displacement magnitude of the subsidiary steerable wheels is derived arithmetically on the basis of the vehicle speed and the steering angle displacement magnitude of the primary wheels. The subsidiary steerable wheels are associated with a subsidiary steerable wheel actuation mechanism including a pulse motor which is driven in response to a subsidiary steerable wheel control signal to cause toe angle change.
Typically, the vehicle speed is monitored by means of a vehicle speed sensor which is adapted to monitor rotation speed of driving wheel for deriving the vehicle speed based thereon. This manner of derivation of the toe angle change magnitude is effective as long as the driving wheel speed substantially corresponds to the vehicle speed. On the other hand, a difficulty is encountered in deriving the toe angle variation magnitude based on the driving wheel speed and the steering angle of the primary steerable wheel when the driving wheel speed does correspond to the vehicle speed. For instance, while an anti-skid brake control is active, the wheel speed vibratingly fluctuates according to skid control cycle of the anti-skid control.